Conventional metal detectors often fail when detecting metal contaminants in products that incorporate metal as part of the product or the associated packaging. Examples of such products include foods encased in a metal foil, foods residing in aluminum cups, bowls or trays, foods with metal foil freshness seals, and foods fully or partially covered with thin metal lids. Other foreign materials such as bone, glass and stone may also be present in food items. X-rays have been used to irradiate food items in an attempt to detect foreign matter. An example of such a device is disclosed in U.S. Pat. No. 6,512,812, entitled X-RAY FOREIGN BODY DETECTOR, issued on Jan. 28, 2003 to Watanabe.
In the typical x-ray detector there are multiple settings which are optimized for the detection of a certain class of expected contaminant. When the expected contaminant is metal, which has a relatively high x-ray absorption rate in comparison to the surrounding food item, the level of x-ray emission and the corresponding detector sensitivity can be set relatively easily to take advantage of the contrast between the food and any metal that may be present. However, if such a metal optimized setting is used, then the detection of bone or glass, as well as any relatively thin or small metal objects, is much less reliable. A proposed solution to this problem is to use multiple x-ray sources to irradiate the item undergoing inspection. An example of a multiple source x-ray inspection device is disclosed in U.S. Pat. No. 6,370,223, entitled AUTOMATIC DETECTION OF BONE FRAGMENTS IN POULTRY USING MULTI-ENERGY X-RAYS, issued on Apr. 9, 2002 to Gleason et al.
Existing foreign object detectors typically function by emitting relatively narrow angle x-rays, that is, the emitted x-rays reside within a plane or relatively narrow cone. A narrow angle x-ray source prevents radiation in unwanted directions and is necessary for safety reasons, but such a source consumes a relatively high amount of power and is relatively expensive. Further, the narrow angle is achieved by utilizing longitudinal radiation from the x-ray transmitting tube, which is a relatively small portion of the total available radiation. The lateral radiation emitted by the tube is not utilized. An example of a detection device using a narrow angle x-ray emitter is disclosed in U.S. Pat. No. 5,428,657, entitled X-RAY MONITORING SYSTEM, issued on Jun. 27, 1995 to Papanicolopoulos et al.
All of the devices disclosed in the previously cited patents share a common construction characteristic insofar as the x-ray emitter is placed physically above a moving conveyor and the x-ray sensor is placed below the conveyor. This arrangement necessarily increases the cost of the resulting machine for several reasons. First, an x-ray emitter requires a substantial power source which must be routed to the emitter location. Second, the heat producing x-ray emitter is relatively difficult to cool in an enclosed, elevated location. Third, the x-ray emitter is necessarily spaced relatively far from the object under inspection because it must be well above the aperture through which the conveyor enters and the aperture itself is as large as possible to accommodate larger test items. This mechanical arrangement results in a reduction in the amount of radiation actually entering or impinging upon any item being inspected at fixed flux intensity, thus requiring an increase in the absolute flux density needed to penetrate denser objects. Fourth, the x-ray detectors reside beneath the conveyor where they are subject to additional contamination and are relatively difficult to cool, isolate from vibration and to service.
Additional problems encountered in a real world food processing assembly line process include the accumulation of contaminants on the test item conveying mechanism. These contaminants typically include remnants of the foodstuffs under test as well as lubricants and particulates present in the food processing environment. While the signal processing aspects of existing x-ray detection devices may be quite exotic, the cleanliness and serviceability or the conveyor belt is often primitive and results in downtime that renders the relatively high reliability of the electronics irrelevant to the total real world duty cycle of the machine.